The Devil’s Hole pupfish was among the first species listed under the Endangered Species Preservation Act in 1967, which later became the Endangered Species Act. The fish is found nowhere else on Earth except in a single geothermal pool in the Nevada desert, within a deep fissure known as Devil’s Hole.

In June 1965, a weighted rope was lowered into the water at Devil’s Hole and kept descending in the dark. It reached 932 feet without touching bottom. The rope had been deployed during a search for two young men who had disappeared inside the cave.

The water in Devil’s Hole stays around 92 degrees Fahrenheit. It connects to a large groundwater system, and earthquakes occurring thousands of miles away can cause the water to surge suddenly. Geologists and biologists have studied the site for decades, partly because its ancient rock formations preserve evidence of environmental change over hundreds of thousands of years.

The 1965 incident began when three young men entered the water after dark. Paul Giancontieri, 19, worked at a cafeteria near the Nevada Test Site. David Rose, 20, worked at a Las Vegas casino parking lot and was Giancontieri’s new brother-in-law. Bill Alter, 19, was with them.

Alter eventually returned to the surface. Giancontieri did not. When Rose went back down to search for him, he also disappeared. Alter followed Rose down to about 175 feet before losing sight of him in the darkness.

Rescuers faced a difficult problem. They knew roughly where the men had entered the water but had no reliable way to determine where they had gone inside the cave system. Forty-four rescuers from the National Speleological Society, the U.S. Navy, the U.S. Air Force, and local agencies carried out numerous dives over about three days. Most stayed above 200 feet for safety reasons because the cave was too deep, narrow, and poorly understood to send divers down until they found the bodies.

One experienced diver, James Houtz, made one of the most serious attempts, reaching about 315 feet. His light showed 30 or 40 feet more water below him but no bottom. Beyond that was darkness. Houtz had explored Devil’s Hole many times before the incident and later told reporters he had made 28 trips and more than 300 dives. Even he could not tell rescuers what lay beyond.

Search teams recovered a mask and snorkel and found a flashlight wedged in rock about 100 feet below the surface with its batteries dead. The two men were never found. The official search eventually ended, leaving scattered equipment and unanswered questions about where the cave had taken them.

After the incident, the National Park Service fenced off the site. The broader scientific significance of Devil’s Hole became clearer over time. The pool is not simply a quiet underground pond. It is a window into a vast groundwater system and a habitat that supports an entire species in a single small space.

Devil’s Hole sits in the Amargosa Desert of southern Nevada. From the surface, it appears as a small opening in the ground with a pool of warm water. The water is about 12 feet wide in places, with a reachable surface area of about 70 feet along the cave. The water temperature remains stable at about 92 degrees throughout the seasons.

Beneath the surface, hundreds of feet of limestone continue downward into a cave that has never been fully mapped to its bottom. The National Park Service describes Devil’s Hole as more than 500 feet deep, though the actual depth remains unknown. Professional divers have explored to about 450 feet without finding the bottom.

The cave connects to a groundwater system that extends more than 100 miles to the northeast. Scientists closely monitor its water level, chemistry, temperature, and wildlife. These precautions protect both people and the species that lives there.

The Devil’s Hole pupfish grows to about one inch in length. Males can appear strong blue, while females are olive green. The species has adapted to water that remains near 92 degrees. Its entire wild range is limited to Devil’s Hole. No second natural population exists elsewhere. If this population disappears from the cave, the species would be extinct.

The fish do not use most of the cave. They live mainly in the upper 80 feet and depend most heavily on a shallow rock shelf near the surface, roughly 11 feet by 16 feet in size. Sunlight reaches the shelf during part of the year, allowing algae and diatoms to grow there. The pupfish feed on those organisms and on small invertebrates. The same shallow area is where they lay their eggs.

This makes the cave’s depth almost irrelevant to the fish’s survival. The cave may extend hundreds of feet into the earth, but the biologically important area is close to the surface. That creates a vulnerability. The fish live next to a vast, largely unexplored groundwater system while relying on a single small habitat patch exposed to changes both above and below the water.

When water levels drop, the shelf can become exposed. When earthquakes send waves through the cave, those waves can sweep away algae and disturb eggs. Scientists have observed the effects on the population over time. For decades, numbers fluctuated, sometimes reaching several hundred fish before declining sharply in the late 20th and early 21st centuries. In spring 2024, the population count reached 191. By spring 2025, a survey counted 38 fish after earthquake-related disturbances.

The source of the water in Devil’s Hole is not rainwater. It comes from a regional groundwater system known as the Ash Meadows groundwater system. The National Park Service describes the system as extending more than 100 miles to the northeast, with groundwater entering the system thousands of years ago. A U.S. Geological Survey study identifies the Spring Mountains, about 50 miles east of Devil’s Hole, as the main recharge area.

As groundwater moves through Devil’s Hole, it carries dissolved minerals. Under the right chemical conditions, calcium carbonate precipitates and accumulates as calcite on the submerged walls. USGS researchers have found dense calcite layers more than 30 centimeters thick, deposited continuously over hundreds of thousands of years.

In 1992, researchers extracted a calcite core from the cave walls, called DH11. The core was only 36 centimeters long. Oxygen isotope measurements along that small piece produced what researchers interpreted as a continuous record of ancient temperatures covering about 500,000 years. The differences in oxygen isotopes became trapped in the calcite as it formed. Scientists combined the measurements with uranium-series dating, which uses radioactive decay as a clock.

When researchers compared the Devil’s Hole signal with marine oxygen isotope records and the Vostok ice core record from Antarctica, the general pattern was surprisingly similar. But the dates suggested that some major warming events recorded in the calcite began earlier than the corresponding shifts in traditional marine timescales. One early study found that the last interglacial transition appeared to begin more than 10,000 years earlier than expected from other records.

The finding raised questions about the Milankovitch hypothesis, which holds that changes in Earth’s orbit and axial orientation affect the timing and intensity of sunlight reaching different parts of the planet over tens of thousands of years, and that those orbital cycles are strongly linked to the timing of major ice age cycles. The Devil’s Hole chronology suggested that major warming could begin when calculated changes in incoming sunlight provided no obvious cause.

The original researchers maintained that their chronology posed a serious challenge to a direct Milankovitch interpretation. Other researchers questioned whether the cave record could safely be used as a definitive clock for global climate events. A 2024 study identified uranium movement as a significant issue in some Devil’s Hole calcite, particularly material that was periodically exposed to air as groundwater levels changed. More than 50 previously reported mixed uranium-series ages could be affected, with apparent ages younger by up to 12,000 years.

By that time, Devil’s Hole had become more than a deep pool with an endangered fish. Its walls preserved chemical traces of ancient climate. A small fissure in the Nevada desert had become a laboratory for reconstructing Earth’s climate history.

The 1965 incident demonstrated that the cave’s geometry itself made rescue extremely difficult. In a 1963 USGS investigation, researchers described Devil’s Hole as controlled by a major fault system with heavily fractured carbonate rock around the opening. Rock fragments were already falling inside the cave, ranging from small pieces to boulders 8 to 10 feet in diameter. The report warned that continued movement along faults or ongoing chemical alteration by groundwater could significantly change the shape of the site.

The unstable walls, ceiling, and floor make exploration dangerous. Devil’s Hole is essentially a steep fissure in the earth’s crust controlled by faults, with water occupying the lower part of a structure that has been repeatedly modified by collapses, groundwater, and tectonic movements. Deeper passages become narrower, darker, and harder to navigate. Thick calcite deposits cover much of the submerged walls.

By the time divers reach the deepest explored sections, they are working within a geological structure that was never an easy passage in the first place. Going further would be extremely dangerous. This was made clear in 1965.

The two men who disappeared in 1965 were never recovered, but experienced cave divers and physical evidence suggest what likely happened. Giancontieri probably went deep enough that distinguishing between the main passage and a side passage became difficult in the dark. If he entered a fissure that gradually narrowed and continued forward, finding the opening behind him would have been harder. By the time he turned around, with limited air and almost no visual references, finding the correct route back would have been extremely difficult.

Rose went down after him. He likely took the route he believed Giancontieri had taken, searching deeper passages for his friend or his equipment. Each turn increased the chance of becoming separated from the entrance. Once the surface disappeared above him, there was no second exit.

Alter followed Rose down to about 175 feet before losing sight of him. He returned to the surface and survived. The National Park Service has noted that experienced cave divers consider disorientation or getting lost within a fissure or tunnel a plausible explanation. Experienced divers describe an initial descent, a chamber-like area, then passages that narrow and bend away. At depth, visibility is nearly nonexistent. A diver could be a short distance from the correct route and have no idea where he was.

That may also explain one of the strangest pieces of evidence found during the search: a flashlight tied to a ledge about 100 feet below the surface with its batteries dead. It is unknown who placed it there or why. It may have been part of a diver’s equipment, something deliberately placed, or an object that ended up on the ledge during the search. There is no evidence it was a marker, but it showed that equipment and divers reached places rescuers could not easily track.

More than 40 rescue dives were conducted by military and civilian specialists over several days. The missing men were never found. The problem was not simply primitive technology. The cave’s geometry itself transformed the unknown into more unknowns.

Devil’s Hole also connects to another submerged cave about 200 meters away, known as Devil’s Hole Cave 2. The two sites look separate on the surface but are connected through the same regional groundwater system. Both intersect a large hydrothermal groundwater system estimated at about 12,000 square kilometers, with water moving through the system over very long time periods. Researchers estimate the groundwater transit time from the recharge area to the caves at about 880 years.

The deepest parts of Devil’s Hole remain uncharted. No evidence has been found of any creature or supernatural presence in the cave. The cave itself is dangerous enough without such things. Somewhere beneath the warm, dark water, passages continue past the deepest point humans have mapped. The final depth has not been seen. A dive line disappears into darkness. Then the line ends. The cave does not.